This invention relates to a tilt device for a marine propulsion unit and more particularly to an improved tilt and trim arrangement for marine propulsion units.
As is well known, many forms of marine propulsion units such as the outdrive of an inboard/outboard drive or an outboard motor are mounted for movement relative to the associated watercraft between a plurality of trim adjusted positions wherein the angular disposition of the propulsion unit relative to the watercraft can be changed to provide optimum performance. In addition, it is a normal practice to support the outboard drive so that it can be tilted up to an out of the water condition.
In the larger units, it is frequently the practice to incorporate some form of power device for achieving both the tilt and the trim operation of the outboard drive. Most normally, the power device is comprised of a pair or series of fluid motors that operate on the outboard drive to achieve both tilt and trim operation. In the most normal type of application, a relatively large diameter, short stroke trim fluid motor is incorporated for achieving the trim operation while a smaller diameter, larger stroke fluid motor is employed for achieving the tilt operation. Conventionally, these fluid motors are supplied with fluid under pressure from a common fluid pump through a hydraulic circuit arrangement. In connection with conventional systems, the tilt up operation must be preceded by full trim movement of the trim fluid motor before the tilt fluid motor receives the full pressure from the pump to achieve tilt up operation. This results in relatively slow tilting up operation.
It has been proposed, however, to employ an arrangement wherein the fluid circuitry for the tilt and trim motors includes a selector valve that bypasses the fluid directly to the tilt fluid motor without flowing to the trim fluid motor to achieve rapid tilt up operation. Such selector valves have the function of, in essence, isolating the trim fluid motor from the hydraulic circuit in order to achieve a faster operation. However, this means that the trim fluid motor is hydraulically isolated from the circuitry of the system.
The hydraulic isolation of the trim fluid motor gives rise to certain difficulties. For example, if the selector valve is left in a position wherein the trim motor is isolated, impacts or driving thrusts transmitted from the outboard drive to the trim motor can cause high pressures to be exerted on the trim motor and its seals. These high pressure applications can cause damage to the motor or its seal.
It is, therefore, a principal object of this invention to provide an improved hydraulic tilt and trim arrangement wherein rapid tilt up operation can be achieved through the use of a selector valve but the trim fluid motor is protected from hydraulic impacts by a pressure relief system.
It is a further object of this invention to provide an improved hydraulic tilt and trim arrangement incorporating a selector valve for isolating the trim cylinder under certain circumstances and which also protects the trim cylinder from impacts when it is isolated.